Process for removing iodide compounds from carboxylic acids and carboxylic acid anhydrides

ABSTRACT

In a process for removing iodide compounds from a liquid composition comprising carboxylic acids having 2 to 6 carbon atoms or anhydrides thereof, by passing the liquid composition through a silver, mercury, palladium and/or rhodium-exchanged strong acid cation exchange resin, prior to contacting with the metal-exchanged resin the composition is contacted with a cation exchanger in the acid form to remove at least a portion of the metal ion contaminants in the liquid composition which have been found to displace the silver, mercury, palladium and/or rhodium from the metal-exchanged resin.

This application is a continuation, of application Ser. No. 07/959,149,filed Oct. 9, 1992, now abandoned.

The present invention relates to a process for removing iodidecompounds, e.g. alkyl iodides and the like, from carboxylic acids and/orcarboxylic acid anhydrides. In particular the present invention issuited to purifying acetic acid and/or acetic anhydride prepared by therhodium catalysed, methyl iodide promoted carbonylation of methanoland/or methyl acetate.

It is known that a problem associated with acetic acid and/or aceticanhydride produced by carbonylation of methanol and/or methyl acetate inthe presence of a rhodium/methyl iodide catalyst system is that evenafter distillation, the acetic acid and/or acetic anhydride frequentlycontains small amounts of iodide impurities. Whilst the exact nature ofthese impurities is not known for certain, they probably comprise amixture of iodide compounds such as methyl iodide and other higher alkyliodides e.g. hexyl iodide, hydrogen iodide and iodide salts. Suchimpurities are particularly troublesome since they poison many of thecatalysts which are employed in subsequent chemical conversions of theacetic acid and/or acetic anhydride. A case in point are the catalystsused to prepare vinyl acetate from ethylene and acetic acid which areextremely sensitive to iodide impurities.

Several methods of removing iodide impurities from acetic acid and/oracetic anhydride are known. GB-A-2112394, for example, teaches the useof anion exchange resins. EP-A-0196173 describes the removal of iodideimpurities from non aqueous organic media such as acetic acid by the useof a silver or mercury containing macroreticular strong acid cationexchange resin such as Amberlyst 15 (Amberlyst is a Registered TradeMark).

EP-A-2968584 also describes using silver exchanged macroreticular resinsto purify acetic acid contaminated with iodide impurities.

It has been found that a problem associated with the use ofsilver-exchanged strong acid cation exchange resins such as described inEP-A-0196173 is that the silver may be displaced by metals which may bepresent as contaminants in the carboxylic acid and/or anhydride. Thisdisplacement of silver is undesirable as it may result in a lowering ofthe capacity and/or efficiency of the resin and it may also result inunacceptable product contamination with silver.

The technical problem to be solved therefore is to provide an improvedprocess for the removal of iodide compounds from carboxylic acids and/orcarboxylic acid anhydrides.

Thus, according to the present invention there is provided a process forremoving iodide compounds from a liquid composition comprising acarboxylic acid having 2 to 6 carbon atoms and/or a correspondingcarboxylic acid anhydride thereof, and metal ion contaminants, whichprocess comprises contacting the liquid composition with ametal-exchanged ion exchange resin having strong acid cation exchangesites at least 1% of which are occupied by at least one metal selectedfrom the group consisting of silver, mercury, palladium and rhodiumwherein prior to contacting with said metal-exchanged resin, the liquidcomposition is contacted with a cation exchanger in the acid form toremove at least a portion of the metal ion contaminants.

By using a cation exchanger to remove metal ion contaminants prior touse of a resin having metal-exchanged strong acid cation sites, thedisplacement of silver, mercury, palladium and/or rhodium from themetal-exchanged sites by the metal ion contaminants is reduced.

The metal ion contaminants in the acid and/or anhydride may arise fromcorrosion or the use of reagents in the up stream process. Any metal ioncapable of displacing silver, mercury palladium and/or rhodium from themetal-exchanged resin should be removed at least in part by theacid-form exchanger. Such metal ions may comprise, for example, at leastone of iron, potassium, calcium, manganese and sodium. The metal ioncontaminants may be present typically at less than 1 ppm but higherconcentrations may be present.

The acid-form cation exchanger reduces the concentration of metal ioncontaminants present in the carboxylic acid and/or anhydride totypically less than 100 ppb preferably less than 50 ppb. Theconcentration to which it is necessary to reduce the metal contaminantsdepends upon the capacity of the metal-exchanged, iodide removing resinbed to tolerate the residual metal contaminants within its operatinglife for iodide removal.

Suitable acid-form cation exchangers for removing metal ion contaminantsin the present invention may comprise strong acid cation exchange resinsfor example strong acid macroreticuler resins, for example Amberlyst 15manufactured by Rohm and Haas; strong acid mesoporous resins, forexample Purolite C145 CT145 manufactured by Purolite and strong acid gelresins, for example IR120A manufactured by Rohm and Haas. Chelatingresins and zeolites may also be used.

The liquid composition comprising carboxylic acid and/or anhydride issuitably passed through an acid-form cation exchanger bed at a flow ratesufficient to achieve the desired reduction in metal ion contamination.This flow rate will depend upon such factors as the level of metal ioncontamination, the efficiency and capacity of the cation exchanger andthe like. Suitably, a flow rate of 1 to 40 bed volumes per hour,preferably 5 to 15 bed volumes per hour may be used.

The temperature of the cation exchanger for metal ion contaminationremoval should be suitable to maintain the acid and/or anhydride in theliquid state. A suitable operating temperature is in the range 20° to120° C., preferably 30° to 80° C.

Any suitable pressure may be used for the metal ion contaminationremoval step.

Metal-exchanged resins suitable for removing iodide compounds in theprocess of the present invention include metal-exchanged strong acidmacroreticuler resins for example Amberlyst 15; metal-exchanged strongacid mesoporous resins for example Purolite C145 or CT145 or Bayer K2411and metal-exchanged gel resins for example IR120A in which the metal isat least one of silver, mercury, palladium and rhodium.

The metal occupying the at least 1% of the strong acid cation exchangesites of the metal-exchanged, iodide-compound-removing resin of thepresent invention comprises at least one metal selected from the groupconsisting of silver, mercury, palladium and rhodium. Preferably themetal is silver.

Operating temperature of the metal-exchanged resin will generally bedetermined by the operating range of the resin but should be in a rangeto maintain the acid and/or anhydride in the liquid state, typically 20°to 120° C., preferably 30° to 80° C.

Any suitable pressure may be used for the metal-exchanged resin bedoperation.

The liquid composition having reduced metal-ion contamination issuitably passed through a metal-exchanged resin bed at flow ratesufficient to achieve the desired reduction in iodide compounds. Theflow rate of acid and/or anhydride through the metal-exchanged resin bedwill depend upon such factors as the level of iodide compound impurity,the efficiency and capacity of the resin and the like. Suitable flowrates are 1 to 40 bed volumes per hour, preferably 5 to 20 bed volumesper hour.

The carboxylic acid used in the process of the present invention maycomprise one or more of acetic acid, propionic acid, butyric acid,isobutyric acid, pentanoic acid and hexanoic acid. The carboxylic acidanhydride used in the process of the present invention may comprise oneor more anhydrides of acetic acid, propionic acid, butyric acid,isobutyric acid, pentanoic acid and hexanoic acid. Mixed carboxylic acidanhydrides may also be used, for example the mixed anhydride of aceticand propionic acids. Mixtures of carboxylic acids and carboxylicanhydrides may be used.

Preferably the process of the present invention is used for purifyingacetic acid and/or acetic anhydride. The process of the presentinvention is particularly suitable for purifying acetic acid and/oracetic anhydride prepared by the carbonylation of methanol and/or methylacetate as described, for example in our published European PatentApplication EP-A-0087870, the contents of which are hereby incorporatedby reference.

The iodide impurities present in the liquid composition comprisingcarboxylic acid and/or anhydride may be C₁ to C₁₀ alkyl iodidecompounds, hydrogen iodide or iodide salts. The process of the presentinvention is particularly suitable for removing C₁ to C₁₀ alkyl iodidecompounds such as methyl iodide and hexyl iodide.

The present invention will now be illustrated by reference to thefollowing examples.

Experiment 1 (Comparative Example)

To show the effect of metal ion contaminants on the performance of asilver-exchanged iodide-removing ion exchange resin the followingexperiment was performed.

A silver-exchanged Purolite C145 ion exchange resin (approximately 35%of cation exchange sites in silver form) was operated at 79° C. with afeed of acetic acid to which had been added hexyl iodide equivalent to aconcentration of about 500 ppb iodide and which also contained about 30ppb iodide equivalent of other unidentified iodide compounds. Thefeedrate of acetic acid was initially 5 liquid hourly space velocity(LHSV) and then increased to 10 LHSV. Assuming that all the acid was fedat a 10 LHSV, the time on stream before iodide breakthrough wascalculated to be 950 hours.

This lifetime is shorter than would be expected from the amount ofsilver present on the resin. The resin was examined and found to becontaminated with an amount of calcium ion equivalent to an average of0.7 ppm in the feed. Analysis of silver, calcium and iodide at differentpositions in the resin bed are given in Table 1. The figures show thatsilver is moving down the resin bed, as

                  TABLE 1                                                         ______________________________________                                                 Concentrations % by weight                                                                          Silver                                                  (figures in bracket mol                                                                             Utilisation                                    Position on                                                                            per kg of resin)      (1)                                            Resin Bed                                                                              Silver    Calcium   Iodide  (%)                                      ______________________________________                                        Top of resin                                                                            6.2 (0.57)                                                                             2.85 (0.70)                                                                             2.5 (0.20)                                                                            35                                       bed                                                                           (feedpost)                                                                    One third                                                                               8.2 (0.76)                                                                             2.05 (0.51)                                                                             1.7 (0.13)                                                                            17.1                                     from top                                                                      Two thirds                                                                             14.9 (1.38)                                                                             1.65 (0.41)                                                                             0.6 (0.05)                                                                            3.6                                      from top                                                                      Bottom of                                                                              13.1 (1.21)                                                                             0.24 (0.06)                                                                              0.1 (0.008)                                                                          0.6                                      resin (outlet)                                                                ______________________________________                                         (1) defined as iodide concentration/silver concentration × 100% on      molar basis.                                                             

calcium accumulates at the top of the resin bed.

Experiment 2

A silver exchanged Bayer K2411 resin (about 35% of cation exchange sitesin silver form) was operated at 79° C. with a feed of the samecomposition as Experiment 1. Acetic acid was fed at a LHSV of 10. Forthe first 1200 hours of operation the concentration of silver in theacetic acid leaving the bed was <30 ppb. The concentration of silverthen steadily increased such that after 1800 hours it was 350 ppb andafter 2200 hours it was 700 ppb. After 2200 hours the resin was examinedand found to be contaminated with calcium ion equivalent to an averageconcentration in the feed of 0.6 ppm for the duration of the experiment.Analysis of silver, calcium and iodide at different positions in theresin bed is given in Table 2.

                  TABLE 2                                                         ______________________________________                                                 Concentrations % by weight                                                                          Silver                                                  (figures in bracket mol                                                                             Utilisation                                    Position on                                                                            per kg of resin)      (1)                                            Resin Bed                                                                              Silver    Calcium   Iodide  (%)                                      ______________________________________                                        Top of resin                                                                            3.9 (0.36)                                                                             4.9 (1.23)                                                                              2.5 (0.19)                                                                            53                                       bed                                                                           (feedpost)                                                                    One third                                                                               6.9 (0.64)                                                                             3.6 (0.90)                                                                              3.0 (0.24)                                                                            37                                       from top                                                                      Two thirds                                                                             13.5 (1.25)                                                                             1.6 (0.40)                                                                              2.4 (0.19)                                                                            15                                       from top                                                                      Bottom of                                                                              19.0 (1.76)                                                                             0.6 (0.15)                                                                              0.4 (0.03)                                                                             2                                       resin (outlet)                                                                ______________________________________                                         (1) defined as iodide concentration/silver concentration × 100% on      molar basis.                                                             

Again, as in Experiment 1, the silver has migrated down the bed. In thiscase it has been so severe that unacceptable levels of silver haveleached into the liquid acetic acid during the latter part of theExperiment.

Experiment 3

To show the benefits of removing metal ion contaminants fromiodide-contaminated acetic acid prior to removing the iodide impuritiesusing a silver-exchanged resin the following experiment was performed.

Acetic acid to which had been added hexyl iodide equivalent to anaverage of 1105 ppb iodide, 170 ppb iron, 75 ppb potassium and whichalso contained an average of 430 ppb calcium, (not added) was passedthrough two sets of resin beds at 79° C. at a LHSV of 10.

Experiment 3A used 30 ml of silver-exchanged Purolite C145 similar toExperiment 1 only and is a comparative experiment.

Experiment 3B used 30 ml of silver-exchanged Purolite C145 similar toExperiment 1 and upstream of this was 30 ml of Purolite C145 in the acidform. This is according to the present invention.

The iodide breakthrough occurred after 5190 hours for Experiment 3A andafter 5000 hours for Experiment 3B.

The silver-exchanged resins were analysed for silver, potassium ion andiodide but by a different technique to that used for Experiment 1 whichdid not allow for calcium determination. The results are shown in Table3.

The results in Table 3 show that the resin bed in Experiment 3B has lessmetals than in Experiment 3A such as potassium and iron, these havingbeen removed by the acid-form resin pre-bed. The lifetime of thesilver-exchanged resin bed without the acid-form resin bed wascomparable with that of the resin bed with the acid-form resin pre-bedbecause the concentration of metal ion contaminants was rather low, sothat the resin bed could accommodate their build-up within the lifetimedictated by its silver loading. Similarly, the distribution of silverwithin the two resin beds was not significantly affected. Had the metalion contaminants been present at higher concentrations it is expectedthat in the absence of the acid-form resin pre-bed they would have had aadverse effect on the lifetime of the silver-exchanged resin bed foriodide compound removal. This is illustrated by the shorter lifetimes ofthe silver-exchanged resin beds in Experiment 1 compared to those ofExperiment 3 (low metal ion contaminants).

                                      TABLE 3                                     __________________________________________________________________________    Concentration % by weight              Silver                                 (figures in brackets mol per kg of resin)                                                                            Utilisation                            Position in                                                                         Silver    Potassium                                                                           Iron  Iodide     (1) (%)                                Resin Bed                                                                           3A   3B   3A 3B 3A 3B 3A   3B    3A                                                                              3B                                   __________________________________________________________________________    Top of bed                                                                          7.6 (0.70)                                                                         7.7 (0.70)                                                                         0.42                                                                             0.04                                                                             1.8                                                                              <0.6                                                                             6.5 (0.51) 8.0                                                                     (0.63)                                                                              73                                                                              89                                   (feedpoint)                                                                   One third                                                                           8.0 (0.74)                                                                         8.3 (0.77)                                                                         0.33                                                                             0.02                                                                             1.5                                                                              <0.5                                                                             6.5 (0.51)                                                                         10.6 (0.83)                                                                         69                                                                              108                                  from top                                                                      Two thirds                                                                          7.7 (0.72)                                                                         8.4 (0.78)                                                                         0.32                                                                             0.02                                                                             1.2                                                                              <0.5                                                                             5.9 (0.46)                                                                         5.8 (0.46)                                                                          65                                                                              59                                   from top                                                                      Bottom of                                                                           8.5 (0.79)                                                                         9.5 (0.88)                                                                         0.21                                                                             0.02                                                                             <0.7                                                                             <0.7                                                                             2.6 (0.21)                                                                         1.0 (0.08)                                                                          26                                                                              9                                    resin bed                                                                     __________________________________________________________________________     (1) defined as iodide concentration/silver concentration × 100% on      molar basis                                                              

We claim:
 1. A process for removing iodide compounds from a liquidcomposition comprising a carboxylic acid having 2 to 6 carbon atoms oran anhydride thereof, and metal ion contaminants .Iadd.selected from thegroup consisting of iron, potassium, sodium and manganese.Iaddend.,which process comprises .Iadd.first .Iaddend.contacting said liquidcomposition with a .Iadd.cation exchanger in the acid form to remove atleast a portion of said metal ion contaminants, and then contacting theresulting product with a .Iaddend.metal-exchanged ion exchange resinhaving strong acid cation exchange sites at least 1% of which areoccupied by at least one metal selected from the group consisting ofsilver, mercury, palladium and rhodium . .wherein prior to contactingwith said metal-exchanged resin, said liquid composition is contactedwith a cation exchanger in the acid form to remove at least a portion ofsaid metal ion contaminants.!.. . .
 2. A process as claimed in claim 1in which said metal-ion contaminants comprise at least one metal ioncontaminant selected from the group consisting of iron, potassium,calcium, sodium and manganese..!.
 3. A process as claimed in claim 1 ..or claim 2.!. in which said carboxylic acid comprises acetic acid.
 4. Aprocess as claimed in claim 1 . .or claim 2.!. in which said iodidecompounds comprise C₁ to C₁₀ alkyl iodides.
 5. A process as claimed inclaim 3 in which said iodide compounds comprise C₁ to C₁₀ alkyl iodides.6. A process as claimed claim 1 in which said acid-form cation exchangercomprises at least one resin selected from the group consisting ofacid-form strong acid cation exchange macroreticular, mesoporous and gelresins, acid-form chelating resins and acid-form zeolites.
 7. A processas claimed in claim 1 in which said metal-exchanged resin comprises atleast one resin selected from the group consisting of macrorecticular,mesoporous and gel resins.
 8. A process as claimed in claim 7 in whichat least 1% of the strong acid exchange sites of said metal-exchangedresin are occupied by silver.
 9. A process for removing C₁ to C₁₀ alkyliodides from a liquid composition comprising acetic acid or an anhydridethereof, C₁ to C₁₀ alkyl iodides and metal ion contaminants.Iadd.selected from the group consisting of iron, potassium, sodium andmanganese, .Iaddend.which process comprises the steps of:(a) passingsaid liquid composition through a resin bed comprising acid-form strongacid cation exchange resin at a flow rate of 1 to 40 resin bed volumesper hour and at a temperature in the range 20° to 120° C. to remove atleast a portion of said metal ion contaminants, and (b) passing theproduct of step (a) through an ion exchange resin bed comprising an ionexchange resin having strong acid cation exchange sites at least 1% ofwhich are occupied by silver, at a flow rate of 1 to 40 resin bedvolumes per hour and at a temperature in the range 20° to 120° C.